The BT Voyager* is an ADSL router which is used on a BT line. If you already have a cable modem from Ntl then you will need a cable router.
If you are wanting to run VOIP without the computer on then you can even get routers with a phone socket built in.
Something like the draytec would be good for you from what I've read about it.
http://www.draytek.co.uk/products/vigor2100v.html
*
http://www.rocom.co.uk/top/computer/BT-Voyager.htm